This invention relates to an antilock control system for vehicle wheel brakes.
When the brakes of a vehicle are applied, a longitudinal (or braking) force is generated between the wheel and the road surface. This force is dependent upon various parameters, including the road surface conditions and the amount of slip between the wheel and the road surface. The braking force increases as slip increases, until a critical slip value is surpassed. When the slip exceeds this critical slip value, the braking force at the tire-road interface decreases and the wheel rapidly approaches lockup. The braking forces at the front and the rear wheels together contribute to the total braking force on the vehicle.
As the wheel travels over the road surface, a lateral force may also be generated between the wheel and the road surface. The available lateral force is maximum when there is no wheel slip present and decreases as wheel slip increases. Thus, the lateral force capability of the wheel is maximized when there is no wheel slip present. Increased lateral force capability at the front wheels contributes towards better steerability of the vehicle; while lateral force capability at the rear wheels contributes towards better stability.
Therefore, to obtain an optimal compromise between the objectives of lateral stability, steerability and improved stopping distance, an antilock braking system must be able to effectively trade-off the longitudinal and lateral characteristics.
When the vehicle is braked on a uniform surface while moving in a straight line, the tire-road friction characteristics for all four wheels are similar. In this case, the longitudinal forces on the right hand side of the vehicle and those on the left hand side are nearly equal. Consequently, the force imbalance, if any, is small and can usually be compensated by the lateral forces at the rear wheels. Hence, little or no driver corrective steering action is required to maintain directional stability.
The longitudinal and lateral forces of the wheels are also key factors when the vehicle is operating on a split-coefficient surface. Such a surface is often encountered during normal driving conditions, such as when the vehicle has the right hand side on a soft gravel shoulder while the left hand side is on asphalt. In such a split coefficient situation, the braking force on the higher coefficient (i.e. asphalt) side of the vehicle will be substantially greater than the braking force on the lower coefficient (i.e. gravel shoulder) side of the vehicle, which causes an imbalance of forces. If the lateral forces of the rear wheels are not great enough to counteract the force imbalance, a net yaw moment tending to rotate the vehicle about its vertical axis results. This incipient yaw condition requires the driver of the vehicle to perform corrective steering in order to maintain directional stability.
There are known systems which attempt to detect yaw moment and take corrective action to minimize its build up when performing antilock brake maneuvers on a split coefficient surface. These systems typically make use of devices such as lateral accelerometers, steering position sensors and other auxiliary devices to sense the yaw condition. Once sensed, a typical antilock brake control system then acts to slow down the additional build-up of the yaw moment such as by commanding a lower rate of increase in brake pressure during the apply portion of an antilock braking cycle as compared to the rate of increase commanded on a uniform surface. The effect of this action is to reduce the imbalance between the longitudinal forces on the two front wheels and thereby slow down the build-up of the yaw forces.
However, the use of auxiliary devices in these systems to sense the yaw condition tends to increase the system cost and complicate assembly and service operations. Therefore, it would be preferable for an antilock system to be able to recognize and counteract an incipient yaw condition without auxiliary hardware or devices. Further, the control action in response to the detected yaw condition is in the form of an open loop control. The lower rate of increase in the brake pressure during the apply portion of the antilock braking cycle may be appropriate for one road surface condition but may not be appropriate for all braking surfaces.